Properties affecting thermal transport are important parameters in the simulation of plastic processing techniques such as injection molding, thermoforming, or extrusion, to name only a few such techniques. In order to simulate such processes as accurately as possible, and hence, ultimately improve the productivity of manufacturing plants utilizing these processes, it is important to determine these parameters as accurately as possible. An example of the importance of the accuracy of these parameters was shown in a study wherein it was determined that in high volume manufacturing processes such as injection molding, a small cycle time reduction based on a more accurate simulation could result in substantial improvement in productivity [Lobo, H. And Newman, R. (1990) Thermal Conductivity of Polymers at High Temperatures and Pressures" SPE-ANTEC'90, 862-865, which is hereby incorporated by reference in its entirety].
In the process of injection molding, the thermal conductivity of polymers and the thermal contact resistance at the polymer-metal interface are two important parameters that govern the heat flow from the molten polymer. Generally, thermal conductivity measurements of polymers are performed using both steady state and transient methods, while thermal contact resistance measurements are conducted by steady state methods. A steady state method generally involves heating a sample to a constant temperature, and then taking data on the temperature of a specific material surrounding the sample at a specific distance from the interface of the sample and the material. These data can then be used to calculate thermal conductivity. In a transient method, the temperature of a material surrounding the sample is taken as a function of time while the temperature of the sample either increases or decreases. Examples of steady state methods presently used to determine the thermal conductivity of a sample include the guarded hot plate method, and methods using heat flux meters [Holman, J. P., and Gadja, W. J. (1984). Experimental Methods for Engineers. McGraw Hill, New York, which is hereby incorporated by reference in its entirety].
Further, examples of transient techniques used to determine thermal conductivity include, but are not limited to the line source method for determining thermal conductivity of polymers (Lobo and Cohen, 1990).
As explained above, experimental methods for determining thermal contact resistance are generally performed using steady state methods (Mohr, J. W., Seyed-Yagoobi, J., and Price D. C. (1997). "Thermal Contact conductance at a Paper/Elastomer Interface." Journal of Heat Transfer, 119, 363-366; Narh, K. A., and Sridhar, L. "Measurement and Modeling of Thermal Contact Resistance at a Plastic Metal Interface." ANTEC '97, Toronto, Canada, 2273-2277; which are hereby incorporated by reference in their entireties] or a quasi-steady state method [Moses, W. M., and Johnson, R. R. (1989). "Experimental Results for the Quasi steady Heat Transfer Through Periodically Contacting Surfaces." J. Thermophysics, 3(4):474-476, which is hereby incorporated by reference herein in its entirety] using temperature measurements, taken at a predetermined distance from the interface, to extrapolate to the interface temperatures, and to calculate the heat flux crossing the interface.
However, steady-state methods of determining thermal heat conductivity contain inherent limitations. Initially, they are very time consuming in that the sample must initially be brought to a steady state prior to determining its thermal conductivity or thermal contact resistance. Moreover, the surfaces of the sample must be "conditioned" prior to determining the thermal conductivity. Conditioning the sample involves keeping the sample at the desired constant temperature for a period of time, usually at least 25 minutes prior to bringing the sample to the steady state temperature. This procedure prepares the surfaces of the sample for determining the thermal conductivity of the sample.
Another drawback in the use of steady state methods to determine the thermal contact resistance is that it does not permit the determination of such resistances at two interfaces, i.e. where opposite surfaces of a sample contact the surface of a second material. Rather, calculations used in steady state methods inherently provide only the average thermal contact resistance of both surfaces of the sample.
Another limitation of steady state methods for determining thermal conductivity or thermal contact resistance involves changes in the physical shape of the sample as a result of heating, which can alter or skew thermal conductivity or thermal contact resistance measurements. In particular, it as been concluded that thermal contact resistance at an elastomer-elastomer interface depends on the flatness rather than on microscopic asperities of the surface of the sample (Mohr et al. 1997). Hence at lower contact pressures of the interface, deviation from the surface flatness causes much larger interfacial gaps, and hence, larger thermal conductivity values than those due to microscopic asperities, which can result in an inaccurate determination of such values.
Hence, what is needed is a transient method of determining thermal conductivity and thermal contact resistance of a sample at an interface with another material, and hence accurately determine thermal contact resistance and thermal conductivity in an efficient and economical manner.
What is further needed is an approach to calculating thermal contact resistance and thermal conductivity which permits the simultaneous determination of these values.
The citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application.